I retired when the Normal and Compulsory retirement ages were both 55, but I was keen to retire and get on with the rest of my life - on a great pension. I also grabbed the opportunity (post 9/11) of 3 years of 50% part time before retiring.
I suppose that 55 year olds now will no longer be near the top of the bidline so will be tempted to stay on for a chance to cream off the best trips.
There was talk (way back) that those electing to stay on after retirement age should start losing seniority, instead of gaining more (to encourage those with full pensions to retire). I don't suppose that ever happened?